Resonances occur when a bigger planet has influence over a smaller body, where objects with orbits that you would thing could collide (same inclination), they in fact, cannot. Iwas just messing around, and I got this neat 2:1 resonance of Earth with a Jupiter. The Earth orbits once for every two orbits of Jupiter.
Shift + F can be used to find resonances with a rotating from to the selected body and it's parent.